PROJECT SUMMARY Shoulder osteoarthritis (OA) is a common shoulder pathology that primarily affects older individuals. In the setting of shoulder OA, the soft-tissue padding within the joint wears down resulting in bone-on-bone contact, which causes inflammation and pain. The resulting pain can be debilitating, limiting daily living and work activities. Treatment for end-stage disease involves total shoulder arthroplasty (TSA), in which the worn, bony surfaces are replaced with prosthetic implants. In shoulder OA, bony erosion occurs symmetrically (concentric deformity) or asymmetrically (eccentric deformity) about the glenoid center. Individuals with eccentric bony deformities who undergo TSA face higher rates of implant failure requiring a revision surgery when compared to the overall TSA population. Revision surgery is costly and results in increased complications and inferior patient outcomes. To date, it has been proposed that the increased revision rates result from inadequate correction of bony deformities; however, advances in surgical technique have not reduced failure rates. A recent clinical theory is that imbalances between agonist and antagonist rotator cuff muscles contribute to bony deformity development and persist following surgery. While this is a novel hypothesis, research has yet to comprehensively quantify muscular balance in individuals with eccentric and concentric deformities prior to and following TSA. This proposal will focus on identifying differences in muscular balance between individuals with concentric and eccentric deformities prior to and following TSA by quantifying strength balance (relative strength in various directions) in individuals with shoulder OA. Further, this work will evaluate the two main determinants of muscle strength: muscle activation and muscle volume. Aim 1 will quantify three-dimensional strength balance across the shoulder in multiple directions during maximal isometric torque production. Aim 2 will quantify shoulder muscle activation (extent to which nervous system can access muscles? full capacity) during submaximal isometric torque production. This aim will also quantify rotator cuff muscle volume, adjusting for muscle degeneration, which serves as an indirect measure of muscle capacity. Strength, muscle activation, and muscle volume will be assessed in individuals with concentric and eccentric bony deformities cross-sectionally prior to and following TSA. For Aim 1, a six degree-of-freedom load cell will be used to measure three-dimensional shoulder torque. For Aim 2, surface and intramuscular electromyography will be used to measure muscle activity about the shoulder. Muscle volume and intramuscular fat will be computed from magnetic resonance images. The results of this work will provide the first quantitative comparison of muscular balance in individuals with concentric and eccentric bony deformities prior to and following TSA. The findings will directly inform current management of individuals with shoulder OA, as the discovery of muscular imbalances would necessitate incorporation of rehabilitation to target specific deficits in strength.